The present invention generally relates to the transmission of digital signals and more specifically to a circuit for shaping digital pulse data transmissions, such as non-return to zero (NRZ) type digital transmissions.
Digital data transmissions are often in the form of a series of transmitted pulses wherein each pulse is transmitted at an amplitude of one of at least two binary states. Such transmissions are often referred to as amplitude shift keyed transmissions and generally are associated with pulses that are transmitted with one of two amplitude levels.
One example of a data transmission format for transmitting digital information is known as the non-return-to-zero (NRZ) format. The NRZ format is a binary amplitude shift keyed format. FIG. 1 illustrates an exemplary NRZ digital transmission comprising a plurality of serial data pulses or "symbols" 10, each of which has a symbol period or width T. Since the NRZ format is a binary data format, the data symbols 10 may have one of only two amplitude states.
FIG. 2 illustrates the frequency spectrum of a single data pulse of the exemplary digital pulse transmission. As shown, the frequency spectrum is centered at a frequency F.sub.c which, for example, may be at baseband or at the frequency of a modulated carrier. The frequency spectrum includes a main lobe 14 followed by multiple side lobes, such as at 16 and 18, that decrease in magnitude along the frequency axis. The information carried by the digital data pulse is principally found in the main lobe 14 having a bandwidth f, where f equals 1/T, and where T equals the pulse width of a single symbol. It is therefore preferable to filter out the frequency components above the frequency f because the side lobes 16 and 18 above frequency f have substantial amplitudes and thus, contain a significant amount of high frequency energy. Consequently, complex, high order filters would nornally be required to filter out an acceptable amount of the high frequency energy. It is, however, undesirable to use complex filters.
It has been proposed to "smooth" the waveform of an NRZ data pulse stream prior to transmission. As shown in FIG. 3, the NRZ digital data stream has been modified to include transition regions, such as 22 and 24, which are substantially sinusoidally shaped. FIG. 2 illustrates the frequency spectrum associated with a single pulse of the waveform of FIG. 3. As shown at 25 of FIG. 2, the frequency spectrum includes a main lobe 26 followed by side lobes 28, 30 and 32. The side lobes 28, 30 and 32 are significantly smaller in amplitude than the side lobes 16 and 18 associated with a non-smoothed NRZ data bit stream (FIG. 1). Consequently, a much simpler filter may be used to filter out signals at frequencies above frequency F.sub.1 prior to transmission.
"Smoothing" of a digital data pulse transmission has several advantageous effects. First, the reduced bandwidth means that more such transmissions can be transmitted within a given allocated bandwidth. This, in turn, means that more intelligent information can be transmitted in a given bandwidth.
One system of note in connection with such an approach to digital data transmissions is set forth in U.S. Pat. No. 4,339,724 titled "Filter" by Dr. Kamilo Feher. The '724 patent seeks to prevent intersymbol interference and jitter, while reducing the bandwidth of the data signals. The '724 patent discloses a filter that includes an input for receiving a pulse type of input signal and for providing an output signal correlated to the input signal. The filter comprises means for comparing the output signal with the input signal and four waveform generators. The first waveform generator produces a first predetermined output signal waveform when the input and output signal amplitudes differ and the input signal equals a logical 1. The second waveform generator produces a second predetermined output signal waveform when the input and output signal amplitudes differ and the input signal equals a logical zero. The third waveform generator produces a third predetermined output signal waveform when the input and output signal amplitudes equal and the input signal equals a logical 1. The fourth waveform generator produces a fourth predetermined output signal waveform when the input and output signal amplitudes equal and the input signal equals a logical zero. The first through fourth waveform generators correspond to a sine wave generator, a cosine wave generator, a positive DC signal generator and a negative DC signal generator, respectively. Each signal generator is turned on and off by switches controlled by the aforementioned logic.
However, the filter of the '724 patent is unduly complex as it requires separate waveform generators to produce each desired segment of the output signal, along with a complex logic and switching network to analyze the input signal and turn on and off corresponding signal generators. The filter of the '724 patent draws a significant amount of power to drive the multiple waveform generators that construct the output signal.